build: adopt uv and PEP 621 packaging

[Claudio9701]

Summary

• replace setup.py, duplicated requirements, pytest configuration, and the stale conda environment with one PEP 621 pyproject.toml
• commit a cross-platform uv.lock and Python 3.11 local baseline
• define test, lint, docs, release, security, and aggregate development dependency groups
• expose urbanpy.__version__ from installed package metadata
• restore compatibility with the supported OSMnx 2.x import path

Validation

• uv sync --locked --python /opt/homebrew/bin/python3 --all-groups
• package import on CPython 3.14
• all 15 tests collect
• sdist and wheel build
• twine check dist/*
• check-wheel-contents dist/*.whl
• Ruff on changed Python files
• git diff --check

Stack

Base: #61
Next: hermetic tests and uv-based mandatory PR checks (will update the legacy workflow that still references requirements.txt).

Closes #73
Refs #63